Most television sets are provided with a bleeder for the high voltage power supply, wherein, after the set has been switched off, the current to the screen and the light output from the screen quickly drop to zero. However, the bleeder adds significantly to the cost of the set and some sets are not provided with bleeders and thus the charge remains in the filter capacitor and the high voltage may linger on for minutes or even hours. Beam currents of non-thermionic origin may then excite the phosphors in the screen. This phenomenon is referred to as "afterglow" and is objectional.
Typically 30% of manufactured tubes may show afterglow. Repeated high voltage processing can bring this figure to below 5%, still an unacceptably high reject rate. Therefore, there is a need for improved cathode ray tubes that do not show any afterglow.